Soyfoods May Help Reduce Risk of Heart Disease and Overall Mortality

More than 40,000 scientific papers focused on soyfoods, soy protein or other soybean components such as isoflavones, have been published over the past 30 years. That research continues with new insights on health impacts of soy.

One area that continues to be heavily researched is cardiovascular disease (CVD). Uncovering the relationship of diet to CVD is especially important since in 2016, more than 1000 deaths caused by heart attack, stroke, or other cardiovascular events occurred daily in the United States. [1]  Nearly half of American adults 20 years of age and older have CVD, which includes coronary heart disease, heart failure, stroke and hypertension. [1]

The good news is that an estimated 70% of major CVD events in the United States are attributable to low and moderate cardiovascular health, based on National Health and Nutrition Examination Survey 2011 to 2016 data from 7 U.S. community-based cohort studies. [1]

Cardiovascular health refers to core health behaviors including smoking, physical activity, and diet, and health factors such as blood cholesterol levels, blood pressure, glucose control, and weight.  This means that leading a heart-healthy lifestyle can dramatically reduce the chances of experiencing a heart attack or stroke.

So, where do soyfoods fit in? Evidence has shown that soy protein favorably affects cholesterol levels,[2], [3] blood pressure, [4] and glucose control, [5] although the findings are not totally consistent. One recent study from China is particularly relevant because it found that eating even amounts of soy was associated with a decreased risk of death and cardiovascular events such as heart attack. This is the first study to examine the relationship between soy intake and total CVD events and all-cause mortality in a Chinese population.[6]

The China Cardiometabolic Disease and Cancer Cohort [4C] study involved nearly 100,000 Chinese men and women over the age of 40. Average age was 56.2 and 66% of the subjects were women. This is one of the largest studies involving soy ever to be conducted. It is also the first soy study to involve participants from all parts of China, rather than from just urban areas such as Shanghai.

During the slightly more than three years the study participants were followed, there were 2523 total CVD events and 1473 deaths from any cause. Total CVD events include being hospitalized or treated for heart failure, non-fatal stroke, non-fatal heart attack, or death related to CVD.

Dietary intake was assessed via a food frequency questionnaire that included 16 items. (Most studies of this type include considerably more items than this, so that is a limitation of this research.) To assess soy intake, participants were asked whether they regularly consumed soy during the year prior to study enrollment and if so, how much and how often. For each participant, total soy consumption was referred to as the sum of all dietary soyfoods consumed per day and expressed as firm tofu equivalents.  Note that there are several types of tofu – extra firm, firm, soft and silken.  The protein content of tofu varies with extra firm containing the most and silken the least.

Participants were divided into four soy intake categories. While it’s commonly perceived that soy is a large part of Chinese diets, 51% of the participants consumed less than 15 grams of soy per day, which is the equivalent of less than 1/5^th of a serving of tofu. At the other extreme, 12% of the population consumed at least 60 grams or more of soy per day or the equivalent of nearly one serving of firm tofu. Compared to those with intakes below 15 grams, subjects with the highest intakes had a 14% lower risk of having a CVD event and a 17% lower risk of death. In both cases, the findings were statistically significant, meaning it’s unlikely they occurred by chance.

Since this is an observational study, it doesn’t provide information about why soy was associated with a reduced risk of CVD. It is possible that people who ate more soy also led overall healthier lifestyles, and the lifestyle, not the soy, was responsible for the reduction in CVD events and death.  However, because soy is a traditional food in China, it’s less likely to be tied to any particular lifestyle. And, to eliminate this possibility as much as possible, the researchers controlled for 17 factors that affect risk including age, sex, education, urban/rural residence, healthy diet, total energy intake, smoking status, drinking status, physical activity, body mass index, systolic blood pressure, HDL-cholesterol (“good cholesterol”), LDL-cholesterol (“bad cholesterol”), total cholesterol, triglycerides, and fasting plasma glucose. Even after eliminating all potentially confounding variables, soy was still associated with a reduced risk.

But by controlling for these variables, this study may have actually underestimated the potential benefits of soy because it eliminated some of the ways in which soy may reduce risk of CVD. What then, accounts for the beneficial effects? The authors of this research noted that soy contains several components that may offer protection against CVD, including isoflavones, phytosterols, lecithin, soluble fibers, saponins, and polysaccharides. Of course, the fat in soybeans is comprised predominately of polyunsaturated fat, which when replacing saturated fat in the diet, reduces blood cholesterol levels.

However, based on current understanding, it is unlikely that any one of these components were consumed in sufficiently large quantities to independently affect risk.  This is why the comment by the authors that these soy components may act collectively to confer unique health benefits is particularly insightful. Increasingly, nutrition scientists talk about the influence of overall dietary pattern on health, rather than the influence of any particular food or nutrient. [7], [8]  The reductionist approach to understanding diet and health has fallen out of favor. What is clear from the China Cardiometabolic Disease and Cancer Cohort study [1] is that soy can be part of an overall dietary pattern that may reduce the risk of the number one killer of Americans. [9]

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2. Blanco Mejia S, Messina M, Li SS, Viguiliouk E, Chiavaroli L, Khan TA, Srichaikul K, Mirrahimi A, Sievenpiper JL, Kris-Etherton P, et al. A meta-analysis of 46 studies identified by the FDA demonstrates that soy protein decreases circulating LDL and total cholesterol concentrations in adults. J Nutr 2019;149:968-81.
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